The invention relates generally to optical data storage and specifically to an apparatus and method for writing data to an optical storage medium such as a DVD or a CD. The invention also relates to a method and apparatus for generating zoned write clocks.
"Read/write" drives can write data to "read/write" optical discs. Read/write optical discs include discs that allow new data to be written only once and optical discs that allow new data to be written many times. A DVD+RW disc is an example of a read/write disc that allows new data to be written many times.
A typical read/write drive includes a spindle motor for spinning the disc and an optical pickup unit for reading and writing data to the disc. The read/write drive is typically operated in a constant linear velocity mode. As the optical pickup unit is moved along the disc in a radial direction, the angular rate of the disc is adjusted so that data is read out at a constant linear velocity.
It is more desirable to operate the read/write drive in a constant angular velocity (CAV) mode. A read/write drive that spins the disc at a constant angular velocity allows data to be accessed faster because the angular velocity of the disc is not repeatedly increased and decreased.
It is even more desirable for the read/write drive to operate in a CAV mode and store the data on the disc at a constant bit density. Resulting is data storage at a constant linear density (CLD). Maximum storage capacity can be achieved if the data is stored at a constant linear density.
Operating a read/write drive in a CAV mode while storing data at a constant bit density can be problematic. Frequency at which new data is written to the disc is constantly adjusted to compensate for the varying radial distance of the optical pickup unit relative to the center of the disc.
A typical solution involves the use of zoned constant angular velocity. Different write clock frequencies are assigned to different zones of the disc. As the optical pickup unit enters a zone, the new data is written to the disc at a frequency assigned to the zone.
However, frequency of the newly written data varies within each zone, and abrupt changes in frequency occur from zone to zone. During readback of the newly written data, the variations and abrupt changes in bit density might cause problems for clocks and data recovery circuitry. Consequently, the variations and abrupt changes might render portions of the read/write disc effectively unreadable by the read/write drive or by another drive.
The problems resulting from abrupt variations may be overcome by the use of "edit gaps" (also known as a "splice areas"). The new data is written to the disc in blocks, and the blocks of newly written data are separated by the edit gaps. The edit gaps allow sufficient time for the clocks and data recovery circuitry to recover from abrupt frequency changes before data is read from the next block.
However, the use of edit gaps has its drawbacks. Storage capacity of the read/write disc is reduced because data is not stored in the edit gaps.
Moreover, the edit gaps might render portions of the read/write disc unreadable by existing read-only drives. Older read-only drives that were developed prior to newer generation read/write drives might not be capable of processing the edit gaps. Unless an existing read-only drive is somehow modified to navigate past the edit gaps, it will have difficulty reading the data stored on the read/write disc.
There is a need for a read/write device that can operate in a CAV mode and write new data to an optical disc at a relatively constant bit density. There is also a need for such a device that does not rely upon edit gaps to overcome the problem arising from abrupt frequency changes in newly written data.